I have a master's degree in English, and have some experience in TESOL. I was an adjunct for an ESL Listening and Speaking class at a local community college. I've done private ESL tutoring for corporate employees and their spouses, and I currently teaching online lessons through a Chinese company.

I live in the United States, and I understand that most adult or post secondary jobs for ELLs will be part time. But will going back to school to pursue a TESOL cert. open anymore doors for me? I am not talking about one of those diploma mill websites, but rather a 4 year public institution that offers a certificate for the completion of 12 graduate hours (4 courses) in ESL. Since I already have an MA, I am not looking to complete the entire degree program, but rather take some courses where I can learn more about teaching methods designed specifically for English Language Learners.

So, considering my education and experience, would there be any advantages to completing one of these programs?

You have no formal teaching qualifications, so that's what's missing from your portfolio, if that is your career choice. The type of teaching qualification required depends upon whether it is a primary, secondary, college, or university environment, in which you want to teach.

Nevertheless, a TESOL certificate, for a native speaker, is deemed to be a sufficient qualification to teach English to non-native speakers. So having a TESOL certificate, of any sort, will definitely open doors for you to teach English as a foreign language. However, the better the institution, the better the TESOL course will be and as you want to improve your teaching techniques, then the graduate course is the way to go.

Part of a TESOL course will be the practical element, which you already have and so all you need now is the theory.